Micrel – High-efficiency BLDC motors: key design considerations

In recent years, there has been a move to the use of Brushless DC (BLDC) motors in many markets and applications, sometimes to replace AC induction motors.

The benefits of using BLDC motors include higher efficiency, reduced heat generation, and higher reliability, since there are no brushes to wear out. BLDC motors are also safer, because they generate no brush dust or arcing. Moreover, BLDC motors can reduce a system’s weight.

Inside the motor

BLDC motors are synchronous motors with permanent magnets attached to the rotor, and coil windings in the stator, as shown in Figure 1. The stator windings produce magnetic fields which can be made to rotate. Precise control of the currents through the windings via a commutation scheme ensures that the generated magnetic field interacts with the permanent magnets’ fields so as to generate torque that spins the rotor.

Fig. 1: Cross-section of a BLDC motor

This calls for switching circuits which can produce bi-directional currents in the stator windings. These switching circuits consist of a highside and a low-side switch for each winding. A total of six switches are needed for one BLDC motor, as shown in Figure 2.

Fig. 2: A BLDC motor’s switching circuit using IGBTs

How a commutation scheme works

For the three windings, a BLDC motor has three phases, and each phase has a conducting interval of 120°. Since the current is bidirectional, each phase is broken into two steps for each conducting interval in a control scheme known as six-step commutation, see Figure 3. One option for sequencing the phases is AB-AC-BC-BA-CA-CB. Only two phases conduct current at any time, leaving the third phase floating.

Fig. 3: Six-step commutation

As described above, the magnetic field in the stator advances ahead of the rotor. For optimal torque, the stator’s magnetic field should be as nearly orthogonal to the rotor’s
magnets as possible, which means that the transition from one sector to another must occur at precisely gauged rotor positions.

The algorithms for controlling the switching of the stator current normally run in a microcontroller. But to drive the MOSFETs or IGBTs, the low-voltage signal from the MCU must be amplified by gate drivers that have the appropriate characteristics, such as low propagation delay, and low rise and fall times, together with sufficient voltage capability to drive the switch safely. The MIC4605 from Micrel is particularly well suited to this task in battery-powered applications such as portable power tools and kitchen equipment.

The rotor position is the key to determining the right instant for commutating the motor winding. In applications where precision is required, Hall sensors or tachometers are used to calculate the position, speed and torque of the rotor. When cost is the most important factor, back Electromotive Force (EMF) may be used to calculate position, speed, and torque.

This means that BLDC motors typically operate in a closed-loop control system, in which the MCU performs the servo-loop control, calculations, corrections, and management of sensors such as back EMF, Hall sensors or tachometers.

An 8-bit MCU might be sufficient, but often 16- or 32-bit devices are used. They require EEPROM to store the firmware that performs the algorithms required to set the desired motor speed and direction, and to maintain the stability of the motor. MCUs often include an adequate ADC that allows for sensorless motor control, which saves cost and board space.

The MCU offers scope to optimise the algorithms for the application. But some external analogue peripherals are also required, as shown in Figure 4. These analogue devices provide the MCU with an energy efficient power supply, voltage regulation, voltage references, and the capability to drive MOSFETS or IGBTs while offering fault protection.

This combination of digital and analogue technologies allows the designer to realise an efficient three-phase BLDC motor, and to achieve a comparable bill-of-materials cost to that of an induction or brushed DC motor.